The proper wiring of the vertebrate brain represents an extraordinary developmental challenge, with over 100 billion neurons forming an estimated quadrillion synapses in humans. An essential but poorly understood feature of this process is the specificity of neuronal connections. Individual neurons must make synaptic contacts with a specific set of target cells in order for neural circuits to be functional. Owing to the stunning complexity of the human brain, simple vertebrate systems must be employed in order to comprehend synaptic specificity in mechanistic detail. The lateral-line organ of larval zebrafish is a sensory system composed of superficial hair cells that transform water-borne mechanical stimuli into electrical activity in afferent nerve fibers. In order to generate real-time estimates of water current velocity and acceleration, the larval brain keeps track of hair-cell location and directional sensitivity with respect to the bodily axes. This function requires that afferent neurons make distinctions based on these two parameters when innervating hair cells. The specific aims of this research are to understand how individual afferent neurons selectively synapse with particular hair cells according to their location and directional sensitivity and to decipher the logic by which these hair-cell parameters are represented in the brain. These experimental aims will be addressed through a combination of genetic and optical imaging techniques. Transgenic approaches that either silence synaptic activity in hair cells or allow for remote control of synaptic activity using channelrhodopsin-2 will provide a means of assessing the role of synaptic activity in specifying appropriate synaptic targets. Analysis of synaptic wiring in a mutant zebrafish lacking the stereotyped pattern of hair-cell polarity will complement these studies. Finally, genetic and optical methods will be developed to map the neural circuits that represent lateral-line stimuli in the brain. The studies proposed here have the potential to shed light upon the establishment of synaptic specificity in a vertebrate sensory system in vivo. Because these specific patterns of neuronal connectivity form the basis for normal brain function, abnormalities in this wiring can cause neurological and psychiatric disease. A detailed understanding of the mechanisms that promote the proper wiring of the nervous system will bolster our understanding and treatment of mental illnesses such as autism, epilepsy, and schizophrenia, as well as our capacity to harness normal developmental processes toward the recovery of brain function following stroke and traumatic injury.